Scientists have long known that many of the early dinosaurs, the ancestors of today's birds, were covered in feathers, probably by heat and to attract partners. But no one knows exactly when – and how – these feathered dinosaurs took off. Now, molecular evidence of fossils of feathered dinosaurs reveals how the major feathery proteins have become lighter and more flexible over time as wingless dinosaurs have evolved into flying ones – and later, birds.
All modern terrestrial animals with spine have keratins, proteins that make up from nails and beaks to scales and feathers. In humans and other mammals, alpha-keratins form the 10 nanometer filaments that make up hair, skin and nails. In crocodiles, tortoises, lizards and birds, beta-keratins form even narrower and more rigid filaments that build claws, beaks and feathers.
Using the entire genome of dozens of live birds, crocodiles, turtles, and other reptiles, scientists have built a genealogical tree of these animals over the last decade based on how their beta-keratins changed over time. Among the revelations: modern birds have lost most of their alpha-keratins, but beta-keratins in their feathers have become more flexible, thanks to a lack of glycine and tyrosine amino acids that make the claws and beaks stiff. This suggests that the transition to flight required both changes to occur.
Now the researchers have shown this directly, analyzing the alpha and beta-keratins in a handful of exceptionally preserved fossils from China and Mongolia. The researchers, led by paleontologists Pan Yanhong of the Chinese Academy of Sciences in Beijing and Mary Schweitzer of the State University of North Carolina, designed separate antibodies to bind to segments identified by various feather-preserved alpha and beta-keratin proteins fossilized. five species that lived between 160 million and 75 million years ago. Antibodies were labeled with fluorescent tags that light up whenever they bind to their targets.
The penalties of AnchiornisA crow-sized dinosaur that lived 160 million years ago, illuminated to reveal the truncated and flexible beta-keratin found in modern birds, researchers report today. Annals of the National Academy of Sciences. But the dinosaurs – which predate the first recognized bird, Archeopteryx, in 10 million years – had even more alpha-keratins, which nowadays are practically absent from bird feathers. Given that, in addition to the structural differences revealed by the electron microscopy analysis, it is likely that Anchiornis The feathers were not suitable for flight, says Schweitzer, but represent an intermediate stage in the evolution of flight feathers.
Fossilized feathers of a small flying dinosaur 130 million years old called Shuvuuia (which is not an ancestor of today's birds) reveal that, like modern birds, alpha-keratins were lacking. But in reverse AnchiornisHis feathers were still composed of the larger and more rigid beta-keratins. "We are beginning to discover the mosaic pattern of the evolution of feathers," says Schweitzer, who suggests that the transition from feathers to flight required mutations that eliminated most alpha-keratins and flexible, truncated beta-keratins.
"This kind of work is the dream of every evolutionary biologist," says Matthew Greenwold, an evolutionary biologist at the University of South Carolina in Columbia who helped build the family tree for beta-keratin. Taken together with modern genetic evidence, the new finding suggests that during the transition to flight, the beta-keratin gene was often duplicated in the genomes of some dinosaurs. As the animals evolved, some extra copies were transformed into the truncated form that made the flight possible. This not only allowed feathered dinosaurs, but also Archeopteryx to cross the sky 150 million years ago, but also gave rise to all the crows, finches, starlings and eagles that we have today.